Ambient lighting (interior lighting) by means of light-emitting diodes (LEDs) has gained a foothold in modern motor vehicles. This type of lighting is realized with luminous devices that can emit multi-colored light. In a luminous device of this type, one or a plurality of differently colored LEDs are arranged on a printed circuit board so that the varying intensity of the illumination results in secondary colors. The varying intensity of the illumination is caused by a varying current feed of the individual LEDs.
An example of multi-colored LEDs is so-called red-green-blue (RGB) LEDs. In the RGB LEDs, different chips are arranged in a housing. The current passing through the LEDs and/or the chips is limited by the use of series resistors if the current supply is realized by a fixed voltage, such as a vehicle electrical system voltage, for example. In addition, the brightness of each individual chip can be dimmed by a pulse widths modulation (PWM) of the supply voltage.
FIG. 1 shows a configuration of a luminous device according to the conventional technology. The luminous device includes three LEDs 71, 72, and 73. Furthermore, the luminous device includes series resistors 61, 62 and 63, each of which is connected to one of LED 71, 72 and 73 in series. The series resistors adapt the supply voltage to the forward bias of the LEDs. Furthermore, the luminous device includes a micro controller 80, which includes switches 81, 82, and 83. By switching the switches 81, 82, or 83 on or off, the supply voltage can be modulated for each LED by modulating the pulse widths.
With modern ambient lightings, the brightness is adapted with a microcontroller in combination with an LED driver at the actual RGB-LED on one and the same printed circuit board. In that case, an RGB-luminous printed circuit board is a full control device. In a normal case, there are a plurality of such luminous printed circuit boards in a vehicle (up to 45). In some vehicles, the printed circuit boards are connected via a data bus (such as a local interconnect network (LIN), for example). This facilitates an individual response of each individual printed circuit board so that, depending on the driving situation, individual printed circuit boards can change their color and/or intensity.
FIG. 2 shows a PWM control of the LEDs 71, 72, and 73 according to conventional technology. The PWM control shown in FIG. 2 generates a secondary color. The PWM control signals for the LEDs start at the beginning of a PWM period and have different lengths of time.
Because of the multiplicity of LEDs installed in modern vehicles, the size of the printed circuit board plays an important role. In particular, the printed circuit board should be dimensioned as small as possible. This is also done for the purpose of further cost efficiency.